1. Field of the Invention
The present invention relates to a sawtooth wave forming circuit, and it particularly pertains to a sawtooth wave forming circuit in which a differential amplifier is utilized.
2. Description of the Prior Art
The theoretical diagram of a conventional circuit for forming a trapezoidal wave, which is a kind of a sawtooth wave, is shown in FIG. 1. Resistances 1 and 2 determine the bias, while a resistance 3 sets the value of the constant current. Then, a transistor 4 operates in such a way as to charge a capacitor 5 with the constant current set by the resistance 3. With resistances 6 and 7 determining the bias, and a resistance 8 setting the value of the constant current, a transistor 9 operates in such a way as to discharge from the capacitor 5 the constant current set by the resistance 8. Numerals 10 and 11 denote switches for alternately connecting capacitor 5 for the charge-discharge cycle, and 12 denotes a D.C. voltage source. In this circuit, when the switch 10 is closed, while the switch 11 is opened, the constant current I.sub.1 is charged into the capacitor 5; on the other hand, when the switch 11 is closed, while the switch 10 is opened, the constant current I.sub.2 is discharged from the capacitor 5.
Now, assuming the values of resistances 1, 2, 3, 6, 7 and 8 respectively to be R.sub.1, R.sub.2, R.sub.3, R.sub.6, R.sub.7, and R.sub.8, and neglecting h.sub.FE of the transistors 4 and 9, the constant currents I.sub.1 and I.sub.2 are given by: ##EQU1##
However, because two circuits for supplying the constant currents I.sub.1 and I.sub.2 are provided, it is difficult to have identical values of constant currents I.sub.1 and I.sub.2 due to the disparity among the resistances 1, 2, 3, 6, 7 and 8, when forming them in a semiconductor integrated circuit. The slopes of the rise and fall of the trapezoidal wave obtained are different.